It is known for electric power supply grids or systems to use protection devices, such as circuit breakers, which serve several functions that are needed in order to ensure the correct operation of the electrical circuit into which they are inserted and the loads connected thereto. For instance, they ensure the availability of the nominal current necessary for several utilities, enable the proper insertion and disconnection of loads from the circuit, and in particular they protect the grid and the loads installed therein against fault events such as overloads and short circuits.
Numerous industrial solutions for the aforementioned devices exist for this purpose. The most typical and traditional solution uses one or more pairs of contacts, which are coupled or separated by mechanical or electromechanical means so as to break or restore the flowing current.
Other more advanced protection devices include automatic circuit breakers provided with an electronic protection unit to activate, in case of a fault, a kinematic chain causing the separation of the circuit breaker contacts.
It is known that during a fault event in the power grid it is particularly important to obtain a very high intervention selectivity of the above mentioned protection devices.
The term “intervention selectivity” or in short, “selectivity”, is understood to mean the capability of isolating from the power supply relatively limited portions of the grid which are close to the fault location.
In other words, the term “selectivity” indicates the capability to operate in a coordinated and limited way, in order to reduce the inefficiency caused by the fault, limiting the inefficiency only to the portions of the grid effectively at risk and thus preserving, at the same time, the operative status of the rest of the system or grid.
On the other hand, it is known that obtaining a high level of selectivity is a particularly difficult aspect in the system design. In fact, it is necessary to reconcile the requirements of better intervention reliability with the obvious need to contain the costs of building and managing the electric grid.
In traditional power grids, a certain selectivity level is obtained by distributing the protection devices throughout hierarchical levels differentiated from a power based point of view (upstream levels, that is closer to the power source are traditionally considered as belonging to a higher hierarchy), and selecting the tripping characteristics for each protection device according to the hierarchy level occupied by the device.
Given that every branch of the grid is dimensioned to safely conduct sufficient electrical power to supply all of the lower levels connected to it, selectivity is obtained by exploiting the differentiation in the tripping parameters (trip current, tripping time, contact mechanical inertia, etc.) existing among the protection devices belonging to different hierarchy levels.
In this way, the inefficiency caused by a fault event only affects the portion of the grid governed by the protection device having the hierarchy level immediately above the level or position where the fault itself occurred.
Some of the technical solutions of the known type provide for the use of protection devices of the so called EFDP (Early Fault Detection Prevention) type, with the capability to communicate between one another, in order to coordinate the respective intervention modalities in case of a fault event.
These established solutions are designed according to selectivity and intervention logic of the bottom-up type, for example, when an EFDP device detects a fault, regardless of its position on the grid, it generates an interlock signal which is sent to all the other EFDP devices located upstream on the grid, inhibiting their intervention capability.
If the EFDP device in question is not itself interlocked by another EFDP device located downstream, it directly operates breaking the current.
Even though the solutions of the known type allow performing the functionality for which they were designed reasonably well, there is nonetheless room and need for further improvement.